Nuclease-resistant phosphorothioate analogues of oligodeoxynucleotides were synthesized by sulfurization of either internucleoside phosphite linkages, in a repetitive manner during chain extension, or internucleoside hydrogen phosphonate linkages, in a single step following chain assembly. These analogues were tested as antiviral agents against human immunodeficiency virus (HIV). In a cytopathic effect inhibition assay using HIV-uninfected susceptible T-cells (tetanus-toxoid specific normal T-cells) co-cultured with irradiated chronically HIV-infected cells, phosphorothioate oligomers inhibited the cytopathic effect and replication of several isolates of HIV-1 and HIV-2. Thus phosphorothioate analogues of oligodeoxynucleotides could inhibit cell-to-cell transmission of the virus as well as the infection by cell-free virus particles and also could inhibit variety of isolates of human retroviruses.